A variety of platforms (e.g., aircraft, automotive vehicles, ships, spacecraft, and buildings) have payloads (e.g., mirrors, telescopes, lasers, cameras, and other types of sensing devices) attached to them that require a vibration free environment. Active isolators have been used to provide the vibration free environment. The isolators perform four different primary functions: connecting the payload to the platform; isolating the payload from the vibrations of the platform; transferring forces from the platform to the payload to change the payload orientation or point the payload; and, transferring forces from the platform to the payload to cancel forces generated on the payload that may induce vibration of the payload.
One example of an active isolator is a passive D-strut in combination with a multistage (e.g., two force coils) actuator. The passive D-strut has a spring and a damper in parallel with the spring along the axis of control of the strut. The spring and damper connect the payload to the bus and make the isolator “soft” (e.g., moveable) along the axis of control and “stiff” (e.g., substantially rigid) in all other axes. Absent a force application (e.g., via the force coil), the spring supports the payload in a neutral position. The damper prevents the payload from being driven into oscillation at the resonance frequency of the spring. A typical force coil is an electrical coil that when energized (e.g., via a force coil drive) applies equal and opposite forces to the payload and the platform along the axis of control. The amount of force the force coil applies is proportional to the magnitude of the current running through the force coil, and the direction of the force application is generally dependent on the polarity of this current.
When using the isolator to prevent the transfer of vibration movements of the platform to the payload, the force coil is driven at a current level that creates the forces for compressing and extending the spring/damper such that the payload sees no change in the force for maintaining the payload in the neutral position. These applied forces are typically in a low force range (e.g., peak forces less than about 1 pound-force (lbf)) over a frequency range of DC to 100 Hz and have a resolution of 0.0005 lbf. These forces are desirably noise/ripple free.
When using the isolator to cancel forces applied to the payload by equipment mounted on the payload, the force coil applies an equal and opposite force to transfer the force to the platform while canceling the force on the payload thereby maintaining the payload in the neutral position. The canceling force is typically less than about 0.1 lbf over a frequency range of about 100 Hz to about 300 Hz and having a resolution of about 0.0005 lbf. This canceling force is also desirably noise/ripple free.
When using the isolator to transfer forces to re-orientate the payload, the forces are generally known and can be directly commanded. These forces are substantially greater in magnitude (e.g., up to about 100 lbf) than the previous mentioned applied forces but are associated with substantially lower frequencies (e.g., less than about 0.01 Hz). Because these forces are used to re-orientate the payload, these forces are not required to be noise/ripple free but should be efficiently generated. However, transitioning from a high-force efficient operation (e.g., for payload re-orientation) to a low force noise free operation (e.g., for canceling forces applied to the payload by equipment mounted on the payload) should be smooth to prevent applying undesirable forces that could disturb the payload and induce vibrations.
One conventional technique for applying some of these forces includes the use of a single force coil driven by a single drive, such as a linear drive or a pulse width modulated (PWM) drive. The single drive can be a single-ended or a dual-ended drive (e.g. an H-bridge drive). The single-ended drive is generally simpler to incorporate a current sense, such as by installing a precision resistor between the force coil and a ground. Additionally, the single-ended drive may be operated accurately by solely supplying a drive current to the force coil and referencing the force coil to ground to minimize common-mode offset. Most conventional drive systems are powered from a single source. Generally, the single-ended drive requires two power sources, a separate power source for each polarity of the single-ended drive, and the second power source would be created to sink and source power for the single-ended drive. The H-bridge drive may be powered from a single power source referenced to ground.
Driving with a linear drive is generally considered noise free due to the constant application of the desired voltage drop to obtain the desired output current/force. However, the linear drive is generally inefficient because, with the continuous application of the required voltage, the linear drive drops the difference between the supply voltage and the desired voltage. With the average voltage drop across the force coil being zero, the total power dissipation is the current multiplied by the supply voltage. The linear drive losses are thus the total power losses less the coil losses.
The PWM Drive applies two different voltages to the force coil over a short time period, and the percentage of time in each voltage state is controlled such that the average voltage applied over this time period substantially equals the desired average voltage over this time period. The PWM drive is generally the most efficient because the output stage of the PWM drive is being switched between two states when the output stage is in saturation. This minimizes the power dissipated in the PWM drive because the power dissipation is the force coil current multiplied by the saturation voltage. In an H-bridge PWM drive, the current flows through two separate drives of the H-bridge PWM drive at all times such that the power dissipation is twice the saturation voltage multiplied by the force coil current.
A digital control system has been used to control the force coil drive with force sensors in active struts, accelerometers on the payload, and an inertial reference sensor system on the payload as disturbance detectors to provide feedback for determining the force coil drive currents. External commands may be provided to the digital control system for repositioning. In this application, the digital control system outputs current commands to the force coil drive via 16-bit digital-to-analog (D-A) converters. Current telemetry feedback is also provided via 16-bit analog-to-digital (A-D) converters. More commonly, 12-bit D-A converters and A-D converters are used for such applications. To control the force coils to adequately output forces for the foregoing operations, the force coils should operate over a range of about −100 lbf to about +100 lbf and with a resolution of about 0.0005 lbf. Dividing the full range (e.g., 200 lbf) by the resolution (e.g., 0.0005 lbf) produces 400,000 increments or just under 23-bits of resolution. The 16-bit D-A of the digital control system has 32,768 bits of resolution.
Accordingly, it is desirable to provide a method and system for driving one or more force coils that may be used with an isolator. More particularly, it is desirable to provide a method and system for efficiently driving one or more force coils over a wide range with higher resolution than presently obtainable. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.